The present invention relates to a method of removing suspended solids from wastewaters, including slurries produced during construction work, e.g. pile driving, tunnel excavation, boring, or dredging of rivers and lakes, and effluents such as activated sludge from, e.g. an industrial plant or a sewage plant.
A huge amount of slurry is produced during pile driving, tunnel excavation, boring, or dredging of rivers and lakes. If such slurry is discharged without adequate treatment, it will pollute rivers and the ground. In a common treatment, the slurry is mixed with a nonorganic or polymer flocculant for flocculation of suspended solids or particles. The resultant floc, after having been settled by its own weight, is dewatered with a centrifugal or press dehydrator for conversion to a cake. In another treatment, the slurry is subjected to time-dependent sedimentation for removal of suspended solids of relatively large size, and then the supernatant portion containing a majority of the suspended solids is mixed with a nonorganic or polymer flocculant. The formed floc is settled by its own weight and dewatered with a centrifugal or press dehydrator for conversion to a cake. As the result, a clarified water separated from the suspended solids of a cake will be discharged.
In the foregoing conventional method, both the sedimentation of large sized suspended solids and the flocculation of remaining suspended solids take a considerable length of time. In addition, the dewatering is not an easy job, and after dewatering, the settled floc in cake form still contains a substantial amount of water and will thus be unfavorable for transportation or disposal. Also, some suspended particles remain in the resultant clarified water after removal of the settled floc produced by flocculation with a flocculant, since the conventional method is far from perfect in removal of suspended particles. Furthermore, the consumption of polymer flocculant is large and the overall cost will be increased.
An improved method has been proposed as depicted in Japanese Patent Laid-open Publication No. 58-51997, in which the concentration of suspended solids is carried out by chemical reaction of multivalent metals with sodium alginate to produce a gel form, and by addition of an organic synthetic polymer flocculant. This method allows the coagulation of suspended solids to be effected within a shorter time and produces a dewatered cake which contains less amounts of water. However, the method is not suited for treatment of a slurry produced during construction work such as tunnel excavation or pile driving.
The construction work associated with amounts of concrete as well as dredged soils commonly produces a slurry which contains a high concentration of metallic ions (e.g. aluminum ions). Such a slurry hardly allows an additive of sodium alginate to be dissolved due to the presence of metallic ions. Even if the additive is preliminarily dissolved in water to produce a solution, it will locally react with the metallic ions upon mixing with the slurry, creating particles of metal alginate. This makes it difficult to physically agglomerate and coagulate the suspended solids.
The improved method disclosed in Japanese Patent Laid-open Publication No. 58-51997 for coagulating the suspended solids to a gel form by reaction of multivalent metal with sodium alginate may be applicable to a specific slurry that does not contain a large amount of metallic ions.